FR3075529A1 - CORRECTION OF AN ICMP PACKET RELATED TO AN IP PACKET HAVING BEEN PROCESSED BY ALG - Google Patents

Abstract

 Method for correcting a packet of an Internet control protocol (ICMP) message, when a packet (12, 12t) is routed between a private network (LAN) and a public network (WAN) having been processed by an application layer gateway (ALG), comprising steps of:  reading (PC1) of a packet of an Internet control protocol (ICMP) message including a “need to fragment” message and a value of a maximum transmission unit (MTU) which defines the size of data packets maximum supported in a communication protocol used by a routing,  calculation (PC2) of a value (n) of a processing size by comparing a size (Lin) of an unprocessed packet (12) by the application layer gateway integrator (ALG) and a size (Lout) of the same packet processed (12t) by the application layer gateway integrator (ALG),  modification (PC3) of the packet of an internet control protocol message (ICMP) by overwriting the value of the maximum transmission unit (MTU) with a value of actual maximum packet size which is a function of the value of the processing size (n),  sending (PC4), to an emitter (1) having sent the unprocessed packet (12t), of the packet of a modified internet control protocol (ICMP) message.

Description

The present invention relates generally to the routing of data in a gateway between a private LAN network (from English: Local Area Network for private network) and a public network WAN (from English: WideArea Network for private network, as opposed to the local area network (LAN), comprising an application layer gateway protocol ALG (for the application layer gateway), for establishing a multi-path connection between the minus computer equipment from a private network and computer equipment or computer network equipment from another public network.

Specifically, the messages conform to the ICMP Internet message control protocol. (ICMP is the acronym for English: Internet Control Message Protocol). The ICMP protocol is used to manage information relating to errors detected by machines connected to the Internet. Thus, the ICMP protocol is used by routers to report an error. ICMP error messages are transported over the network in the form of a datagram, like any data in a packet of an ICMP internet control protocol message.

The invention relates more particularly to a method of correcting a packet of a control protocol message on the Internet ICMP originating from a SIP packet (from English: "Session Initiation Protocol" for session opening protocol) containing a size exceeding the maximum transmission size MTU (from the English: “Maximum Transmission Unit” for maximum transmission unit) of a router, when routing a packet between a private network and a public network by through an ALG application layer gateway integrator.

The invention finds applications, in particular, in calls using the SIP protocol whose packets are processed by an ALG application layer gateway protocol.

Technological background

The ALG application layer gateway protocol is a technique used in computer networks to increase the security of a firewall or NAT between a local network and a public network. The ALG application layer gateway protocol is a software component that manages specific application protocols such as SIP and FTP (from the English File Transfer Protocol). The ALG application layer gateway protocol therefore acts as an intermediary between a public network (Internet) and a private network in a gateway which may include application protocols. For example, the ALG application layer gateway protocol rewrites the SIP protocol layer to allow translation between a private LAN and a public WAN. The ALG appears as the end point before routing the gateway, that is, leaving the gateway. The ALG controls the authorization or refusal of traffic to an application on the server. It does this by intercepting and analyzing specified traffic, to allow traffic to pass through the gateway.

When sending a packet from an internal network to an external network, the ALG application layer gateway protocol can convert information, addresses, into a network layer of the packet. This can cause problems because the packet size can exceed a maximum transmission size of a host, such as a router, between a transmitter and a receiver.

It is known that in case of a packet size exceeding a maximum transmission size MTU of a Host, for example a router, to use an ICMP internet control message protocol which is an error message , for example when a service or a host is inaccessible, sent to the sender. The router uses the ICMP internet control message protocol, to forward a packet of an ICMP internet control protocol message to the transmitter including information about the host's maximum transmission MTU size in occurrence of the router. A flag DF (Don't Fragment) of the outgoing packet allows to discover the Maximum MTU transmission size of a given path (also called "MTU discovery" of the English discovery of the maximum MTU transmission size). Along the path taken by such a packet, if one of the hosts or routers crossed has an MTU smaller than the size of the transmitted packet, the latter will be rejected and a packet of a control protocol message on the ICMP internet, containing an MTU size of the host that generates it, will be sent to inform the source.

An example of this type of message is represented for example in FIG. 6 which is a copy of a part of the computer screen, on which one can read:

type 3 / code 4, Fragmentation Needed the value of the MTU is in a field of the packet of a control protocol message on the ICMP internet: "maximum transmission size MTU of next hop": 1492 in this example

In particular, the usual MTU values are known for different types of network:

over Ethernet:

MTU = 1,500 bytes by default.

MTU = 1500 to 9000 bytes with Jumbo frame.

on PPPoE: MTU = 1,492 bytes.

on ADSL or ATM25: MTU = 1468 bytes.

over ADSL over Wi-Fi MTU = 1468 bytes.

Thus, the transmitter can send a packet, for example of size 1491 bytes and if there is no error message, it can deduce therefrom that it can send at least packets of 1491 bytes.

If the transmitter receives a packet of an ICMP internet control protocol message with the fragment information and including the information of a maximum transmission size MTU of 1468 bytes, the transmitter can either fragment to reach a size less than or equal to 1468 bytes, or reduce its message to reach 1468 bytes, especially when the packet cannot be fragmented.

For example, in the VoIP (Voice over IP) telephone framework, a transmitter can send in the private network a packet marked "do not fragment" DF comprising in the application layer 7 a SIP packet which is an S.I.P. The SIP packet is a signaling protocol with information allowing the initialization of multimedia sessions and can be modified by the ALG application layer gateway protocol.

Indeed, during the processing of the SIP packet by the ALG application layer gateway protocol, the header of the SIP packet can be modified, for example an IP address or a port can be modified, information can be added in the application layer of the SIP packet. This change may result in a change in the size of the IP packet sent as described above.

When a packet exceeds the maximum transmission size MTU of a packet which can be transmitted at once (without fragmentation), a routing sends back to the sender a packet of a control protocol message on the ICMP internet including the information on a fragmenting need and the maximum transmission value of MTU.

In case the maximum transmission size value MTU is less than the size of the IP packet transmitted by the sender, the sender can either send back a smaller packet or can accept the fragmentation of the packet by sending the same packet but without the DF flag.

However, since the ALG application layer gateway protocol has modified the packet, it is possible in certain cases that the value of maximum transmission size MTU is greater than the size of the packet sent by the transmitter and obviously smaller than the same packet processed by the ALG application layer gateway protocol. The transmitter does not understand the error mentioned in the packet of an ICMP internet control protocol message since the size of the packet sent by the transmitter was less than the maximum transmission value.

As the transmitter does not understand, the calling process stops and therefore fails.

An example will be described to better illustrate this problem:

The transmitter sends a packet with a size of 1490 bytes including an IP packet marked "do not fragment" DF (from English: "don’t fragment" not to fragment). The ALG application layer gateway protocol modifies the SIP packet by modifying in particular the LAN IP addresses by the gateway WAN IP address and can modify the port. In this example, these changes result in a 7-byte increase in packet size.

The packet size processed by the ALG application layer gateway protocol is therefore 1490 bytes + 7 bytes, or 1497 bytes. The routing of the gateway after the ALG application layer gateway protocol to pass from the private LAN to the public WAN network can for example accept the size of 1497 bytes, but one of the routings for example, in the public network, does not accepts only 1492 bytes, that is, the value of the maximum transmission size MTU is 1492.

This routing, due to the SIP packet marked DF, will send back to the sender of this packet, a packet of a control protocol message on the ICMP internet marked "fragment needed" with a value of maximum transmission size MTU of 1492 This will pass the gateway from the public network to the private network, then the transmitter will receive this packet of a control protocol message on the ICMP internet containing the value of maximum transmission size MTU of 1492 while the transmitter sent a packet with a size of 1490 bytes.

There is therefore a failure of the call. This is particularly dangerous in the case of an emergency call.

There is therefore a need to remedy these problems.

Summary of the invention

The invention solves a problem posed by an increase in packet size by the ALG application layer gateway protocol and in which the increase in added size exceeds the value of the maximum transmission size MTU. routing between the transmitter and the receiver.

The invention aims to eliminate, or at least mitigate, all or part of the disadvantages of the aforementioned prior art.

To this end, a first aspect of the invention proposes to modify the value of the maximum transmission size MTU received in the packet of a control protocol message on the ICMP internet by a lower value depending on the modification of the size of the packet by ALG application layer gateway protocol.

To this end, a first aspect of the invention proposes a method for correcting a packet of a control protocol message on the ICMP Internet, when routing a packet between a private network and a public network having been processed by an ALG application layer gateway, comprising steps of:

• Reading a packet of an ICMP Internet control protocol message including a Need to Fragment message and a maximum transmission unit value (MTU) which defines the maximum supported data packet size. in a communication protocol used by a routing, • calculation of a value of a processing size (also called length) by comparing a size of a packet not processed by the application layer gateway integrator ALG and a size of the same packet processed by the ALG application layer gateway integrator, • modification of the packet of an ICMP internet control protocol message by overwriting the value of the maximum transmission size MTU with a value of size maximum actual packet which is a function of the value of the processing size, • sending, to a sender who sent the unprocessed packet (12t), the packet of a message of ICMP modified internet control protocol.

This allows the modification size value of a packet modified by the ALG application layer gateway protocol to be taken into account in the packet of an ICMP internet control protocol message.

It follows that in the next test, the transmitter will modify its packet to have a size less than the value of the actual maximum transmission size MTU corresponding to a value of maximum transmission size MTU when the packet is modified. The modification can for example, in the case of a call by the SIP Protocol, be carried out by deleting an encoded code in the message part.

According to one embodiment, the method for correcting a packet of an ICMP Internet control protocol message, further comprising a step of:

• calculation of the actual maximum transmission size value by subtracting the calculated value of the processing size from the maximum transmission unit value of the packet of an ICMP internet control protocol message.

This embodiment allows the packet size that the transmitter can send to be the maximum size before processing. The transmitter can therefore delete a message such as, a code that is not very useful, having a size corresponding at least to the actual maximum transmission size subtracted from the size of the packet sent previously.

According to an embodiment which can be combined with the mode described above, the method for correcting a packet of a control protocol message on the Internet further comprises a step of:

• measurement of the size of the untreated packet, • storage of the size of the untreated packet, • measurement of the size of the packet processed by the ALG application layer gateway integrator, • storage of the value of the size of calculated salary.

According to one embodiment, the method comprises a step of sending a packet of a control protocol message on the Internet ICMP comprising the value of maximum actual transmission size MTU if the size of the packet processed is greater than a size predetermined, for example 1500 bytes.

This allows the actual maximum transmission size value to be sent directly. The predetermined size can be updated in the event that a new known maximum transmission size MTU is greater than this predetermined size.

A second aspect of the invention also relates to a method of processing a packet between a private network and a public network comprising the method of correcting a packet of a control protocol message on the ICMP Internet described above or one or more of the embodiments described above. This method further includes a step of:

• reception of a packet, marked not to fragment, not processed by a gateway integrator of the ALG application layer, • transformation of the packet not processed by the gateway integrator of the ALG application layer into a processed packet .

According to one embodiment of the method for processing a packet between a private network and a public network, the method further comprises a step:

• sending the processed packet to the recipient on a public network if the size of the processed packet is not greater than a size value of a maximum transmission unit (MTU, maximum transmission unit) of the gateway routing.

This embodiment allows the packet to be sent over the public network only if the value of the size of the packet sent is less than the maximum transmission size MTU of the gateway.

According to one embodiment of the method for processing a packet between a private network and a public network which can be combined with the previous embodiment, the method further comprises a step of:

Generation of a packet of a control protocol message on the Internet ICMP if the size of the packet processed is greater than a value size of a maximum transmission unit MTU, and in which the step of modifying the packet d a control protocol message on the Internet is carried out.

This allows direct modification in the gateway routing of the ALG application layer gateway protocol to send the actual maximum transmission size.

According to an embodiment of the method for processing a packet between a private network and a public network which can be combined with the previous embodiments, the message packet is an IPV4 or IPV6 packet.

According to an embodiment of the method for processing a packet between a private network and a public network which can be combined with the preceding embodiments, the application layer of the message sent is a SIP signaling protocol.

This allows a call to be made using an ALG application layer gateway protocol.

According to an example of this embodiment, the SIP packet contains an INVITE or BYE request.

According to one embodiment of the method for processing a packet between a private network and a public network which can be combined with the preceding embodiments, the application layer of the message is an FTP file transfer protocol.

A third aspect of the invention also relates to an ALG application layer gateway protocol integrator comprising the correction method described above with or without one or more embodiments described above.

A fourth aspect of the invention also relates to a "Computer software" product comprising the ALG application layer gateway integrator according to the third aspect of the invention.

A fifth aspect of the invention also relates to a router comprising the "Computer software" product comprising the ALG application layer gateway integrator according to the fourth aspect of the invention.

A sixth aspect of the invention also relates to a private LAN network comprising a router according to the fifth aspect of the invention and at least one device capable of sending a packet to the router, in particular a SIP packet comprising an INVITE request.

A seventh aspect of the invention also relates to a device of an ALG application layer gateway allowing communication between a private network and a public network, in which the gateway device comprises:

• a private data input configured to receive data from a private LAN, • a public data input configured to receive data from a public WAN network, • a data memory for storing data • a memory for program including:

i) a program for reading a packet of an ICMP Internet control protocol message including a need to fragment message and a value of maximum transmission unit (MTU) which defines the size of packets maximum data maximum data supported in a communication protocol used by a routing, ii) a program for calculating a value of the processing size by comparing the size of a packet not processed by the gateway integrator ALG application layer and the size of the same packet processed by the ALG application layer gateway integrator, iii) a write program to modify the packet of an ICMP internet control protocol message by overwriting the value of the maximum transmission unit by an actual maximum packet size value which is a function of the value of the processing size, iv) a sending program to a transmitter having sent the unprocessed packet to the private entry, from the Modified Internet Control Protocol (ICMP) message packet.

According to one embodiment, the device of an application layer gateway is a router.

An eighth aspect of the invention also relates to a computer program product which can be loaded directly into the internal memory of a digital computer, comprising portions of software code which, when said program is executed by a computer, lead the latter to implementing all the steps of the correction method or of the treatment method described above without or with the various embodiments described.

A ninth aspect of the invention also relates to a machine-readable data recording medium comprising a processor, comprising the computer program described above.

Brief Description of the Drawings

Other characteristics and advantages of the invention will become apparent on reading the description which follows. This is purely illustrative and should be read in conjunction with the accompanying drawings in which:

- Figure 1 is a block diagram showing an example of sending a packet having a size greater than the maximum transmission size value MTU of a WAN routing and implementing the correction of the maximum transmission size value MTU in a packet of an ICMP internet control protocol message;

- Figure 2 is a block diagram showing an example of sending a packet having a size greater than the maximum transmission size value MTU of a LAN-WAN routing and implementing the correction of the size value of maximum transmission MTU in a packet of an ICMP internet control protocol message;

- Figure 3 is a diagram showing the case where the transmitter has modified its packet after the case of Figure 1 or 2;

- Figure 4 is a step diagram which illustrates an implementation of the method for correcting the value of the maximum transmission size MTU.

- Figure 5 shows a portion of a screen print illustrating an example of an IP packet not to fragment containing a SIP protocol.

- Figure 6 shows a portion of a screen print illustrating a packet of an ICMP internet control protocol message.

Detailed description of embodiments

In the description of embodiments which follows and in the figures of the appended drawings, the same or similar elements bear the same references in the drawings.

As discussed in the introduction, different Layer 7 applications can be modified by the ALG application layer gateway protocol, including FTP and SIP.

The invention will be described by way of example with the sending of a SIP packet.

The SIP protocol has many applications such as VoIP calling, video calling, instant messaging, virtual reality or even online video games.

SIP shares many similarities with the HTTP protocol such as ASCII encoding and response codes.

In the header, we find information such as:

the source IP address the destination IP address the type of message (INVITE, ACK, OK, BYE, etc.) the type of content of the message (often SDP) etc.

The invention will be described in particular for a VoIP call by the SIP protocol.

The transmitter, also called a client, sends requests to the server, which returns a response. The basic queries are:

• INVITE allows a client to request a new session, • ACK confirms the establishment of the session, • CANCEL cancels a pending INVITE, • BYE ends a session in progress, • OPTIONS allows to recover the user management capacities, without opening a session, • REGISTER allows you to register with a registration server.

The invention will be described with an example of a SIP INVITE package in the following, but of course is only by way of example.

Figure 1 shows an example of sending the SIP INVITE packet.

Figure 5 shows an example of an IP packet using the SIP Protocol including an INVITE request. The SIP packet sent by the transmitter can include for example for an IPv4, by indicating the Flag field bit 1 which says bit DF (Don't Fragment) (on the contrary if it is set to 0, fragmentation is authorized) . In the example described below bit 1 of the Flag field is set to 1 and therefore fragmentation is prohibited. The IP packet in Figure 5 includes the indication do not fragment DF: don’t fragment.

The IP packet in Figure 5 shows that its “Total length” size is 1485 Bytes.

The transmitter 1 which is for example an IP telephone is in a private LAN network and sends a packet 12 of SIP protocol labeled DF having an INVITE request in a private data entry configured to receive data from a private LAN network, d a router 2. Router 2 is a device which serves as a gateway between a private LAN network and a public WAN network. Router 2 therefore includes a public data entry configured to receive data from a private WAN network. By input, we mean data that can enter or exit router 2.

FIG. 4 shows a step diagram of the router 2 showing a step P1 of receiving this packet 12. The packet 12 is intended for a receiver 3 being for example a computer. The router 2 performs a step P2 visible in FIG. 4 which stores the value Lin of the size of the packet 12 which is in this case 1485 bytes in a data memory making it possible to store data.

Router 2 includes program memory including an ALG application layer gateway protocol for adapting packet 12 from the private LAN to a public WAN. The router therefore performs a step P3 of processing the ALG application layer gateway protocol of the packet 12, which causes modifications to the SIP application layer, in particular the occurrences of the LAN IP address which are replaced by another address. IP WAN as well as in this example the modification of a port. The package 12 then becomes a processed package 12t. The modifications increased the size Lin of the packet 12 which goes from 1485 to the size Lout of the packet processed 12t to 1495 bytes in this example.

Router 2 performs a step P4 which stores the size Lout of the processed packet 12t in the data memory. Router 2 then includes a routing E1 which in this case checks the maximum transmission size value MTU which is 1500 bytes and compares it with the size of the packet 12t.

The 12t packet is then transferred to the Public network through which it will go through several routes, in particular E3 routing. The E3 routing also checks the size of the 12t packet compared to the value of its maximum transmission size MTU which is in this case 1492 bytes.

Since the size of the 12t packet is greater than the maximum transmission size of the E3 routing, the E3 routing returns towards the telephone 1 a packet of a control protocol message on the ICMP internet. FIG. 6 shows an example packet of an ICMP internet control protocol message to be retransmitted to the transmitter 1. In this packet of an ICMP internet control protocol message, we can see a field “MTU next hop ”whose value is 1492, corresponding to the maximum transmission size MTU of this E3 routing.

The packet of a control protocol message on the internet passes through the same routers through which the processed packet 12t has passed and in particular passes through router 2 to pass from the public WAN network to the private LAN network.

Router 2 having received the packet of an ICMP internet control protocol message, and having stored the value of the size of the packet 12 and of the same processed packet 12t can carry out a method of correcting a packet of a ICMP Internet control protocol message.

An example of this PC correction method is shown in the step diagram in Figure 4. In this example, changing a maximum transmission size value MTU in the packet of a control protocol message on the internet ICMP is performed when an ICMP internet control protocol message packet is received in the router and the original IP packet has been processed by an ALG application layer gateway protocol. Router 2 therefore includes a program memory for performing this correction process. The correction method comprises a first step PC1 of receiving a packet of an ICMP internet control protocol message and reading in the packet of an ICMP internet control protocol message of a value of maximum transmission unit (MTU) which defines the maximum data packet size supported in E3 routing.

The correction method, in this embodiment, further comprises a step of calculating PC2 of a value of the processing size by comparing the length of a packet not processed by the application layer gateway integrator ALG and the packet size processed by ALG application layer gateway protocol. In this case, the value of the size of the untreated packet 12 was 1485 and the size of the treated packet was 1495, ie a value of the processing size of 10.

The correction method further comprises a step PC3 of modifying the packet of a control protocol message on the internet by overwriting the value of the maximum transmission unit with a value of actual maximum packet size which is a function of the value processing size.

In this case, the modification step PC3 comprises a subtraction of the calculated value of the processing size which was equal to 10 from the value of the field "MTU of next hop" which was 1492 from the packet of a message of ICMP internet control protocol. Thus the actual maximum packet size value in the "maximum MTU of next hop transmission size" field is 1482 of the packet of a modified ICMP internet control protocol message.

Finally, the correction method includes a step of sending to the sender of the packet an ICMP modified internet control protocol message, in which the maximum transmission size field "MTU of next hop" is 1482 and not 1492 .

It follows that the transmitter 1 understands the error of the fact that its packet was 1485 and modifies it for example by deleting an encoded then returns its packet 12m with the SIP protocol and the INVITE request comprising a size less than 1482 bytes, for example 1480 bytes.

FIG. 3 is a diagram showing the case where the transmitter has modified its packet after the case in FIG. 1. The 12m packet is processed by the ALG application layer gateway protocol. A 12mt packet leaves the ALG application layer gateway protocol with a size of 1490 bytes for example, then goes through different routes E1, E2 E3, En until arriving at receiver 3.

Of course, another routing, for example E4 not shown may have a maximum transmission size MTU less than 1492, for example 1468 bytes. In this case, if the IP packet still includes a DF (Don't Fragment) flag, the protocol process for a control message on the ICMP internet starts again, since the size of the new 12mt packet is greater than the maximum transmission size MTU of E4 routing. A new packet of an ICMP internet control protocol message is therefore sent from routing E4 to router 2, including a maximum transmission size field "MTU next hop" of 1468 bytes. Router 2 therefore performs the PC2 calculation step. In this case, the value of the size of the unprocessed packet 12 was 1480 and the size of the processed packet was 1490 to pass the last value of maximum transmission size MTU of 1492 being always the value of the processing size of 10. Router 2 then modifies the maximum transmission size MTU by replacing 1468 with 1458 (1468 - 10).

. It follows that the router performs the step of sending PC5 to the sender 1 of the packet of an ICMP internet control protocol message comprising the value of the maximum transmission size MTU of 1458 in the field "MTU next hop ".

It follows that on each reception of a packet of an ICMP internet control protocol message originating from an IP packet having been modified by the ALG application layer gateway protocol, the PC2 calculation step and PC3 modification step and finally the step of sending the packet of a modified ICMP control protocol message on the PC4 internet. Thus, in the event of an ICMP internet control message protocol originating from a packet modified by the ALG, the IP transmitter always receives a packet of a modified ICMP internet control protocol message. This has the advantage of avoiding a second packet of an ICMP internet control protocol message by the same host (router) since the processing is taken into account from the first time.

Router 2 can also, following the modification of packet 12, sent by transmitter 1, by the application layer gateway protocol ALG send to transmitter 1 a packet of a control protocol message. on the ICMP internet if the size of the modified packet is greater than its maximum transmission size MTU. FIG. 2 represents a functional diagram showing the router implementing the ALG application layer gateway protocol and the packet of an ICMP internet control protocol message.

The transmitter 1 therefore sends an IP packet 12 from a private network for a call by VoIP using the SIP protocol to a receiver 3 via a LAN network as in the example in FIG. 1. The receiver 3 can also be in another private LAN but accessible only by a Public WAN network.

In this example of call, the packet 12 which has a size Lin for example 1491 bytes, has its value of size stored as in the example of FIG. 1 in a data memory then the packet 12 is modified by the protocol of ALG application layer gateway to become a processed 12t packet containing 1501 bytes. Router 2 stores, as in the example in FIG. 1, the size Lout of the packet processed 12t in a data memory. Router 2 performs a calculation of a value (n) of the processing size by comparing the size Lin of the packet 12 and the size Lout of the packet 12t. In this example, the router stores the value n in a “sk_buff” structure of the IP packet. LAN-WAN routing 1 checks the Lout size of the 12t packet of 1501 bytes against its maximum MTU transmission size of 1500 bytes. The 12t packet being a SIP packet marked “do not fragment” DF, the router 2 sends towards the transmitter 1 a packet of a control protocol message on the ICMP internet containing a value of maximum transmission size MTU in function of the value n stored in the IP packet.

In this case, the program memory includes a write program for modifying the packet of an ICMP internet control protocol message by overwriting the value of the maximum transmission unit with an actual maximum packet size value. which is the value of the maximum transmission size MTU of the router or 1500 subtracted from the processing value n which is 10.

The program memory further comprises a program for sending to one of the private inputs or outputs having received from a message sender the packet of an ICMP modified internet control protocol message.

According to another example, the value of the maximum transmission size MTU is modified by the value of the maximum transmission size MTU received subtracted from the sum of the value of the processing size calculated in step PC2 and by a predetermined value. safety margin. The safety margin can for example be equal to 1 byte.

The present invention has been described and illustrated in the present detailed description and in the figures of the appended drawings, in possible embodiments. The present invention is not limited, however, to the embodiments presented. Other variants and embodiments can be deduced and implemented by the person skilled in the art on reading this description and the attached drawings.

In the claims, the term include or include does not exclude other elements or other steps. A single processor or more than one other unit can be used to implement the invention. The various features presented and / or claimed can be advantageously combined. Their presence in the description or in different dependent claims does not exclude this possibility. The reference signs should not be understood as limiting the scope of the invention.

Claims (9)

claims 1) Method for correcting a packet of a control protocol message on the internet (ICMP), during the routing of a packet (12, 12t) between a private network (LAN) and a public network (WAN ) having been processed by an application layer gateway (ALG), comprising steps: • reading (PC1) a packet from an Internet control protocol message (ICMP) including a “need to fragment” message and a value of a maximum transmission unit (MTU) which defines the packet size maximum data supported in a communication protocol used by a routing, • calculation (PC2) of a value (n) of a processing size by comparing a size (Lin) of an unprocessed packet (12 ) by the application layer gateway integrator (ALG) and a size (Lout) of the same processed packet (12t) after processing by the application layer gateway integrator (ALG), • modification (PC3 ) the packet of an internet control protocol (ICMP) message by overwriting the value of the maximum transmission unit (MTU) with an actual maximum packet size value which is a function of the value of the processing size (n), • sending (PC4), to a transmitter (1) having approx use the unprocessed packet (12) of the Modified Internet Control Protocol (ICMP) message packet. 2) Method for correcting a packet of an Internet control protocol (ICMP) message, according to claim 1, further comprising a step of: • calculation of the actual maximum packet size value by subtracting the calculated value (n) of the processing size from the value of the maximum transmission unit of the packet of a control protocol message on the Internet (ICMP). 3) Method for correcting a packet of an Internet control protocol (ICMP) message, according to claim 1 or 2, further comprising a step of: • measurement of the size (Lin) of the untreated package (12), • memorization of the size (Lin) of the untreated package (12), • measurement of the size (Lout) of the same package (12t) after treatment with the application layer gateway integrator (ALG), • storage of the value (n) of the computed processing size. 4) Method for processing a packet between a private network and a public network comprising the method for correcting a packet of a control protocol message on the Internet (ICMP) according to one of claims 1 to 3, comprising further a step of: • reception of a packet (12), marked not to fragment, not processed by an application layer gateway integrator (ALG), • transformation of the untreated packet (12) into a packet (12t) processed after processing by the application layer gateway integrator (ALG). 5) Method for processing a packet between a private network and a public network according to claim 4, further comprising a step of: • generation of a packet of a control protocol message on the Internet (ICMP) if the size of the processed packet (12t) is greater than a value size of a maximum transmission unit (MTU, maximum transmission unit), and wherein the step of modifying the packet of an internet control protocol (ICMP) message is performed. 6) Method for processing a packet between a private network and a public network according to one of claims 4 to 5, in which the message application layer is a signaling protocol (SIP). 7) Gateway comprising an application layer (ALG) allowing communication between a private network and a public network, in which the gateway device comprises: • a private data input configured to receive data from a private network (LAN), • a public data input configured to receive data from a public network (WAN), • a data memory for storing data • a program memory comprising: i) a program for reading (PC1) a packet of an Internet control protocol message (ICMP) comprising a message need to fragment and a value of a maximum transmission unit (MTU, maximum transmission unit) which defines the maximum data packet size maximum data supported in a communication protocol used by a routing, ii) a calculation program (PC2) of a value (n) of the processing size by comparing the size ( Lin) of a packet (12) not processed by the ALG application layer gateway integrator and the size (Lout) of the packet processed (12t) by the ALG application layer gateway integrator, iii) a writing program (PC3) for modifying the packet of an internet control protocol message (ICMP) by overwriting the value of the maximum transmission unit (MTU) with a value of actual maximum packet size which is function of the value (5) of the size of tr aement, iv) a sending program (PC4) to a transmitter that sent the unprocessed packet (12t) in the private entrance, of the packet of a modified Internet control protocol (ICMP) message. 8) Computer program product loadable directly into the internal memory of a digital computer, comprising portions of software code which, when said program is executed by a computer, cause the latter to implement all the steps of the process according to any one of claims 1 to 6. 9) Machine readable data recording medium comprising a processor, comprising the computer program according to claim 8.